1. Field of the Invention
This invention relates to rapidly solidified powder metallurgy aluminum-lithium-zirconium-X alloys, and, in particular, to a process for developing enhanced toughness through temperature control during powder degassing.
2. Description of Related Art
Aluminum-lithium alloys are increasingly important materials for lightweight high stiffness applications such as aerospace components. Rapidly solidified aluminum-lithium alloys having reduced density and improved mechanical properties are disclosed in copending U.S. patent application Ser. No. 478,306, filed Feb. 12, 1990. Those are defined by the formula Al.sub.bal Li.sub.a Cu.sub.b Mg.sub.c Zr.sub.d wherein "a" ranges from about 2.1 to 3.4 wt%, "b" ranges from about 0.5 to 2.0 wt%, "c" ranges from about 0.2 to 2.0 wt% and "d" ranges from greater than about 0.6 to 1.8 wt%, the balance being aluminum. Forgings produced from these rapidly solidified aluminum lithium alloys have significantly improved mechanical properties compared with forgings produced using conventional ingot aluminum lithium alloys. The properties of forgings produced from a similar alloy but having somewhat lower zirconium have been reviewed by Kim, Raybould, Bye, and Das, Proc. Conf. Al-Li V, (1989), pg. 123 and by Quist, Bevers and Narayanan, Proc. Conf. Al-Li V, (1989), pg. 1695. In particular, Quist et al., who represent the perspective of the aerospace industry, have stated that further improvements in the strength-toughness combination are needed before these alloys can find widespread use in aerospace components.
Production of rapidly solidified aluminum lithium alloys can be divided into several steps. In the first step, the alloy is rapidly solidified by melt spinning into ribbon, which is thereafter pulverized into powder. The second step comprises degassing the powder and consolidation thereof into a bulk piece. In the third step, the consolidated article is extruded and/or forged into a useful shape. The fourth and final step comprises heat treating the alloy to optimizing the desired strength and ductility.
The present invention is directed to the degassing step of the process and provides a method whereby certain degassing parameters, especially the degassing temperature, is controlled to markedly improve the final toughness of the alloy. When carried out using alloys having appropriate zirconium levels, the process of the present invention produces Al-Li containing material having a significant strength-toughness improvement.